The role of iron in enhancing phytoplankton productivity in high nutrient, low chlorophyll oceanic regions was demonstrated first through iron-addition bioassay experiments1 and subsequently confirmed by large-scale iron fertilization experiments2. Iron supply has been hypothesized to limit nitrogen fixation and hence oceanic primary productivity on geological timescales3, providing an alternative to phosphorus as the ultimate limiting nutrient4. Oceanographic observations have been interpreted both to confirm and refute this hypothesis5,6, but direct experimental evidence is lacking7. We conducted experiments to test this hypothesis during the Meteor 55 cruise to the tropical North Atlantic. This region is rich in diazotrophs8 and strongly impacted by Saharan dust input9. Here we show that community primary productivity was nitrogen-limited, and that nitrogen fixation was co-limited by iron and phosphorus. Saharan dust addition stimulated nitrogen fixation, presumably by supplying both iron and phosphorus10,11. Our results support the hypothesis that aeolian mineral dust deposition promotes nitrogen fixation in the eastern tropical North Atlantic.
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We would like to acknowledge the captain and crew of the F/S Meteor and the scientists aboard the Meteor 55 SOLAS cruise, especially D. Wallace, K. Lochte, H. Bange, P. Croot, M. Voss, F. Malien, R. Langlois and P. Fritsche. We thank D. Wallace for insightful comments on this manuscript. Additionally, we acknowledge W. Balzer for loaning the clean container used during M55. This work was supported by the Deutsche Forschungsgemeinschaft's Meteor Schwerpunktprogramm, a Natural Environment Research Council grant to R.J.G., and a Marie Curie Post Doctoral fellowship to C.R. Authors' contributions The first three authors made equal contributions to the success of the experiments. This manuscript is the product of an equal collaboration between the groups of J.L. at IfM-Geomar and R.J.G. at University of Essex.
The authors declare that they have no competing financial interests.
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Mills, M., Ridame, C., Davey, M. et al. Iron and phosphorus co-limit nitrogen fixation in the eastern tropical North Atlantic. Nature 429, 292–294 (2004). https://doi.org/10.1038/nature02550
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